<!DOCTYPE html>
<html lang="zh-CN">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /*
      伪元素
       ● ::before 或 ::after
       ● 必须设置 content 属性
       ● 默认是行内元素
     */
    div::before {
      content: '::before';
    }

    div::after {
      content: '::after';
    }
  </style>
</head>

<body>
  <div>Hello world!</div>
</body>

</html>